An apparatus for extracting a hue area, has been proposed, for instance, in Japanese Patent Laid-Open Publication No. HEI 1-114267. According to the technology described in this publication, the hue detection device detects a hue by executing multiplication in response to one having the largest amplitude among a plurality of digital color image signals (R, G, B) and using the image signal having been subjected to the processing for multiplication.
A color image processing apparatus has been proposed also in Japanese Patent Laid-Open Publication No. HEI 1-269365. This color image processing apparatus enables color conversion by determining a hue area to which a color indicated by an image signal belongs, selecting a set of prespecified parameters according to the determined area, and processing the image signal based on the selected parameters.
In the hue detection device as disclosed in Japanese Patent Laid-Open Publication No. HEI 1-114267 and the color image processing apparatus as disclosed in Japanese Patent Laid-Open Publication No. HEI 1-269365, when a hue is detected, a hue area is separated from other areas with a straight line. Namely, a flat and straight plane passing through the origin of the color space is assumed as the border, whether the color has a hue for a specific color or not is detected depending upon on which side of the border a hue of the color is present.
However, in reality, the hue fluctuation in color is in a curved form as shown in the hue fluctuation characteristics (color chart) in FIG. 18. Letters a and b in FIG. 18 indicate axes each indicating density of a specific color in the color chart when the image is read. Because of such characteristics, there is a disadvantage that, when an operator tries to convert a color to a particular color, namely for instance from red to cyan, sometimes the color conversion is not executed in the manner the operator desires.
As understood from the hue fluctuation characteristics shown in FIG. 18, this phenomenon occurs because the hue fluctuation for red in a CMY image from a thin chroma to a dense chroma is not linear.
There is also known also a device which makes it possible to changeably set a hue area to realize the hue conversion in the manner desired by an operator. However, when a hue area is changeably set, the required circuit scale becomes disadvantageously large.